Where to start upgrading?

I have had a headphone system for decades. Over the last ten or fifteen years I finally buckled down to really make it great. It took many years. I went as far as importing a headphone amp as well as buying several others. For headphones the most important component is the headphone amp... period... OK, within reason not a $25 steamer with it. 

The company that changed my perception of what was possible was Woo. They specialize in headphone tube amps. I purchased a WA6 (fantastic but limited power), then the two component WA6SE (fantastic but a bit more power... but limited) and the lots of non-Woo solid state headphone amps. Finally I bought the Woo WA5LE. That amp (see my virtual system) completely changed everything. It is so powerful as to be able to power speakers, it is a 300B tube amp, so is incredibly warm and natural. The combination takes any headphone (especially Sennheiser) and grabs it and tell it who is boss.The power, natural musical sound that comes out is incredible. I have Sennheiser 800, Focal Utopia... etc. In each case they have never remotely sounded as good... not even close. The amazing thing, is when headphones have this kind of midrange bloom and power... they are satisfying from a power perspective... like a big main system is. When I listen to them, I don’t crave going to my main system to get the satisfaction of the bass hitting my chest. 

If you really want to make a leap that will completely change your system. Get a Woo WA5... later upgrade the 300B tubes to Takatsuki. Then upgrade your inputs... each upgrade will be amazing... with WA5 in place. 

I seldom say, "get this". But the way you phrased your question, it seems like the right thing. 

Thank you!  That's exactly the advice I was looking for.  After reading back my initial post, I think I could simplify the question by saying, "if I spend $5,000 on a turntable, will my other components hold me back from noticing a difference".  It makes sense to upgrade the headphone amp to get the most out of the headphones and move on from that point.  

that upgrade path can be sometimes tough to figure out. Used to have nearly the same HP system as the OP. You can start with the table and cart or the HP amp. You should be able to hear the improvements as make the upgrades. think there are two paths you can go. Start with a better HP amp like the quicksilver HP amp. Then go to a technics 1300 or 1200G with the cart of your choice. Instead of using the Denon cart with the Zen phono buy a decent SUT and run the Denon into the MM stage of the Zen. 

There are a lot of wonderful tube headphone amps out there and I've had several - Woo certainly makes some great ones in varying price ranges, and I've had that Quicksilver HP amp that @johnss mentioned; great bang for the buck, that is! But there are also companies like Ampsandsound, Decware, Donald North Audio in the US, and Auris Audio and Feliks in Eastern Europe who make great gear worth investigating (though Decware and DNA have long waiting lists). WE 300B tubes are also excellent.... I even drive my Susvaras and Modhouse Tungstens through some of these relatively low wattage amps and they sound just fine. 

As far as two channel in listening goes, I’d look into acoustic treatments before upgrading components.